In image production systems that produce images on a recording medium, such as printers, photocopiers, facsimile machines and other xerographic devices, it is desired to control, as closely as possible, the actual perceived color of the output images. One known method to optimize image color output is to provide a look-up-table (LUT) that translates received color signals into optimized color signals for printing, for example, on a printer.
It is known, for example, that in three-color spaces, such as a Cyan-Magenta-Yellow (CMY) color space, gray color is made up of equal, or near-equal amounts of each one of the colors of the three-color space. Each color in a three-color space which is made up of non-negligible amounts of all three primary colors of the color space can be viewed as having a gray component. Expanding the three-color space to include Black (K) allows then, for most colors in the color space, for a black (K) component to be added in substitution for the gray component. In such a solution, a three-input, four-output LUT is needed.
Adding black (K) as a fourth color in this manner usually saves cost, as black (K) ink is usually cheaper than colored ink, and allows more colors to be produced than were achievable with the original three primary colors. Controlled amount of black addition is considered useful for high quality printing. Having black gives better stability to prints in the presence of print variables (relative humidity, temperature, material latitude etc.,). Increased gamut for dark colors, as in DC8000, is also achieved with the addition of black toner. One major disadvantage in adding black is the excessive black in flesh tones, sky tones and other important tone scales can make these tone scales appear dirty/grainy or non-uniform with black toner. However, some key colors (e.g., flesh tones and sky tones) are sensitive to the addition of black and may not be perceived as optimal if too much black is added. The replacement of the inherent gray component of colors in a three-color space with a fourth, black (K) component is called gray component replacement (GCR) or under color removal (UCR). UCR is usually used when colors are near the neutral axis, such as, for example, the L* axis in L*a*b* space or the C=M=Y axis in CMY color space. GCR is similar to UCR, but can be used with colors throughout the color gamut, not just near or at neutral axes. The use of GCR and UCR is known to facilitate the production of pleasing color outputs, optimal gamut, and to improve constraints on area coverage.
Traditionally, determination of the black (K) component in a target color system was done in an ad hoc way by experienced practitioners. This method has the disadvantages of requiring experienced personnel, being generally irreproducible, being costly, and being time-consuming.
Another method used to transform colors in a three-dimensional color space, such as CMY color space, to a four-color color space, such as CMYK color space, is to determine the black (K) component by a one dimensional function that relates the black (K) component as a one-dimensional function of the other components. In the CMY color space, for example, the function K=min (C, M, Y) can be used. This method has the disadvantages of not producing sufficiently optimized colors for the entire color gamut, especially for specialized, or key, colors such as, for example, skin tones.
In another method, a flexible method for estimating the black (K) component comprises (1) determining a maximum black (K) component, (2) adjusting the black (K) component amounts based on chroma, and (3) determining the other color components. In examples of this method, disclosed in U.S. Pat. No. 5,502,579 to Kita et al. (Kita '529) and U.S. Pat. No. 5,636,290 to Kita et al. (Kita '290), input image signals are transformed by a four-input-three output controller to L*a*b* color space. The disclosure of each of Kita '529 and Kita '290 is incorporated herein by reference in its entirety. A chroma determining means determines chroma signal C* from a* and b*. A UCR ratio calculation means calculates an UCR ratio α from the chroma signal C*. The L*a*b* and UCR ratio α are then converted into the CMYK output. This method also has the disadvantages of not producing sufficiently optimized colors for the entire color gamut.
In another method, disclosed in U.S. Pat. No. 6,744,531 to Mestha et al. (Mestha), incorporated herein by reference in its entirety, consistent output across multiple devices is obtained. For a given device, received device independent image data are stored as target image data and also converted by a data adjustment subsystem to printable image data based on the color space of the device. The printable image data is printed. An image sensor senses the printed image data and outputs detected device independent image data to the data adjustment subsystem. The data adjusting subsystem compares the detected device independent image data with the stored target image data and, based on the comparison, determines adjustment factors that are used to conform the printable image data output by the data adjusting subsystem to colors mandated by the device independent image data.
In R. Bala, “Device Characterization”, Chapter 5, Digital Color Imaging Handbook, Gaurav Sharma Ed., CRC Press, 2003, several methods for determining the black (K) component are reviewed. One method is black addition in which the black (K) component is calculated as a function of a scaled inverse of L*. In another method, the black (K) component is calculated as a function of the minimum value of the other color components, such as C, M, and Y for the CMY color space. In a third method, a three input-four output transform, subject to imposed constraints, is used to calculate the black (K) component. The constraints placed on the transform include requiring the sum of the color component values at a node to be less than a threshold. For example, in CMYK color space, C+M+Y+K would be constrained to be less than a threshold. A second constraint is to constrain K to be a subset of the range between the minimum and maximum allowed K values.
Another method is discussed in (1) R. Balasubramanian, R. Eschbach, “Design of UCR and GCR strategies to reduce moiré in color printing”, IS&T PICS Conference, pp. 390-393 (1999) and (2) R. Balasubramanian, R. Eschbach, “Reducing multi-separation color moiré via a variable undercolor removal and gray-component replacement strategy”, Journ. Imaging Science & Technology, vol. 45, no. 2, pp. 152-160, March/April, 2001. A UCR/GCR strategy is proposed that is optimized to reduce moiré. In this method, the UCR/GCR strategy is to characterize moiré as a function of the color components and to select optimized output color components when the moiré function is minimized.